But later in life you learnt that subject on your own because you got involved in a project that made you learn the subject. Maybe you came across a very good book or course.
Has it happened with anyone?
A few years out of school I ran across a math problem (not work related) that was interesting enough to dig in to and started hanging out on math stackexchange and later even bought mathematica to help me understand AND solve problems FOR FUN and I was hooked.
A couple years later I had to take a writing class at Oakland University in Rochester, MI, got an A, and the professor tried to talk me into a career in writing. She was a professional writer herself, working for GM.
It's pretty common for students to fail a course and pass it or even master it later.
You just have to put in the work. Finding the right resources absolutely helps as well. As does shoring up your foundational prerequisites.